1. Field of the Invention
The present invention relates to a method and system for tracking the position of a probe attached to a directional drilling assembly. Specifically, the present invention provides a method and apparatus for monitoring the actual location of a drilling assembly during the drilling operation to permit necessary adjustments in drilling direction.
2. Background
Directional drilling refers to a well-known drilling technique whereby the direction of the borehole may be chosen by the operator. Directional drilling has been in use for many years. One application of directional drilling is in conjunction with placing underground transmission lines such as utility lines. communication lines, or pipelines across manmade or natural obstacles. Common obstacles are rivers or other bodies of water, city streets and highways. In order to traverse the obstacle, the drilling assembly must tunnel underneath the obstacle from an above-ground entry point to a desired exit point. The borehole thus created serves to contain the pipeline or other type of transmission line. In many obstacle traversals, the borehole must remain in an assigned right of way as it crosses underneath the obstacle. The point where the borehole exits the ground must also stay within a certain predefined area. Those problems are compounded by the fact that many such underground obstacle traversals cover a considerable distance.
The equipment commonly used to guide a directional drilling assembly as it creates a borehole is a steering tool. Such a steering tool is described in U.S. Pat. No. 3,862,499, the disclosure of which is hereby incorporated by reference. The steering tool allows an operator to change or adjust the orientation of the drilling assembly such that a borehole is created in a corresponding direction. The steering tool itself, however, does not feed back information to the operator indicating the position of the borehole. Thus, the borehole must be directionally drilled in open-loop fashion dependent on the accuracy of the steering tool and land survey data. The accuracy of steering tools in use today, however, is not entirely satisfactory. That lack of accuracy can cause (and frequently does cause) the borehole to deviate greatly from the desired course. Errors in land survey data can also cause such deviation. When the deviation is beyond acceptable limits, which it often is, the borehole must be redrilled. Such redrilling is, of course, costly and time consuming. Even the possibility that redrilling may be necessary is sometimes enough to make some obstacle crossings uneconomical to even attempt. There has been a long-felt need, therefore, for a method and apparatus which would allow an operator to accurately track the progress of a borehole as it is created by a directional drilling assembly such that errors in the borehole's course can be corrected as they occur.
The problem has been addressed in the past through a variety of methods, all of which are largely unsuccessful. For example, methods exist in the prior art which use magnetic fields to guide the directional drilling of a borehole. For example, U.S. Pat. Nos. 3,406,766 and 3,853,185 disclose methods utilizing a magnetic field source which guides the drilling by means of a probe fixed to the drilling assembly for detecting the direction of the magnetic field. Since the absolute location of the probe is not determined by the field measurement, however, the accuracy of each succeeding measurement is dependent on preceding ones. Any measurement errors are thus cumulatively propagated during the entire drilling procedure. One major reason for such measurement errors is magnetic interference from extraneous sources.
U.S. Pat. Nos. 3,529,682, 3,589,454 and 3,712,391 describe a two-loop antenna system for generating two time-varying magnetic fields with differing frequencies. The position and heading of the drilling device is determined by means of magnetometers mounted within the drilling device One of the loops is a dipole antenna while the other loop is a quadrupole antenna. Filters are used to separate the voltage signals produced by each coil which are due to each loop antenna. By measuring the relative and absolute magnitudes of the dipole and quadrupole fields, the depth and lateral displacement of the probe with respect to the two-loop antenna can be determined. The system does not allow, however, for a determination of the probe's position along the length of the drillig path from the magnetic field data. The geometry of the two-loop antenna is also critical for the system to perform properly which can complicate long distance obstacle crossings where the surface environment would not permit a particular loop configuration.
It is an object of the present invention, therefore, to provide a system and method for determining the location of a probe fixed to a drilling assembly from a magnetic field generated by one or more loop antennas constructed of straight segments but otherwise of arbitrary size and shape.
It is a further object to provide a system relatively unaffected by magnetic interference from extraneous sources.
It is a still further object to provide a system whereby each measurement of the magnetic field at the location of the probe is independent of every other, thus avoiding any cumulative propagation of measurement errors.
The present invention permits accurate monitoring of the actual location of the drilling assembly underground during the drilling operation. That information can be used by the driller to make the necessary adjustments in directing the steering tool so that the borehole stays within the right of way and exits the ground within acceptable limits.